Recent United States military missions demonstrated the need for effective and light-weight armor systems that can rapidly respond to a broad range of threats in limited regional conflicts. Ballistic experts in recent years have puzzled over a troubling loss of impact resistance in an extremely hard and lightweight ceramic material called boron carbide, sometimes used in protective armor. The material does an excellent job of blocking low-energy projectiles such as handgun bullets, but shatters too easily when struck by more powerful ammunition.
By observing the atomic structure of boron carbide fragments retrieved from a military ballistic test facility, researchers discovered that the higher-energy impacts cause tiny bands of boron carbide to change into a more fragile glassy form. This high-impact pressure-related amorphyization, or transformation to a glassy material, was previously seen in minerals and semiconductors, and it was also found in a ceramic as hard as boron carbide. The extremely high velocities and pressures associated with impact of a high-powered projectile appear to cause microscopic portions of the crystalline lattice structure of the material to collapse. Based on the analogy of crystalline material, most of modern lightweight armor appliqués are designed in periodic lattice or cellular truss configurations, which provide relatively effective blast energy dissipation strategies.
However, it was also expected that the high intensive wave could cause band instability or collapse in a lattice structure. Numerical simulations proved that the banded failures were formed in the two lattice structures initiated by one broken link (see FIGS. 1(a) and 1(b)). Experiments also indicated that the different orientations of lattice structures could cause different formats of failures in a lattice structure (see FIGS. 2(a) and 2(b)).
Including the waiting links in the structure could improve the situation of banded failure; however, the narrow banded shock waves by the broken links in a conventional triangle or square-cell lattice could still cause high banded deformations as well as banded failures. A series of conventional triangle lattice structures were simulated, and the strain concentration was found on the two ends (see FIGS. 3 and 4). Tiny bands of the whole structure experience total broken, leading to the failure of the whole lattice. Even though the lattice structures with “waiting element” showed the potential as an effective blast assistant armor appliqué, there is still a need to improve the present designs, due to the likelihood of banded failure expected in the conventional lattice structures.